Anomalous Toughening of Glassy Ionic Comb Polymers: Retention of Homogeneity Achieved by Bulky 4-Dimethylaminopyridinium Counterions

Abstract

Overcoming the trade-off between stiffness and toughness is a longstanding challenge for glassy polymers. While ionic toughening with a homogeneous nanostructure offers a potential solution, this behavior remains limited to specific ionic liquid-based systems. Here, we show that incorporating 4-dimethylaminopyridine (DMAP) as a counterion into ionic comb polymers enables effective toughening while maintaining a homogeneous nanostructure. Specifically, the DMAP-neutralized ionic comb polymers demonstrate exceptional toughness (∼137 MJ/m3) and high elastic modulus (∼0.9 GPa)—representing fourfold and twofold increases, respectively, over the non-ionic precursor. SAXS analysis confirmed that these systems maintain a homogeneous nanostructure despite their highly ionized state, as verified by FT-IR. Moreover, DMAP counterions effectively plasticize the polymer matrix, lowering the glass transition temperature (Tg) from 96 to 81 °C. These findings suggest that homogeneous ionic crosslinking, enabled by the combination of bulky side chains and organic bases, is a promising strategy for toughening glassy polymers.